october 14, 2016 [email protected] huntley...

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October 14, 2016 File: 21.0056797.00 Mr. Kevin Schroeder [email protected] Huntley Power LLC Tonawanda, NY 14150 Re: CCR Surface Impoundment Design Criteria Huntley Generating Station Tonawanda, New York Dear Mr. Schroeder: GZA GeoEnvironmental of New York (GZA) presents this letter to Huntley Power LLC (Huntley) with respect to the existing coal combustion residuals (CCR) surface impoundment (identified as the South Settling Pond) located at the Huntley Generating Station in Tonawanda, New York (Site). This Design Criteria information is required by the United States Environmental Protection Agencies (USEPA) Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities; Final Rule, as presented in the Federal Register Volume 80 No 74 dated April 17, 2015. In accordance with the CCR Rule (40 CFR §257.73), owners/operators of existing CCR surface impoundments are required to document the following initial structural integrity criteria: §257.73 (a)(2): an initial hazard potential classification assessment §257.73 (d): an initial structural stability assessment, and §257.73 (e): an initial safety factor assessment The requested structural integrity criteria for the Huntley plants South Settling Pond was recently evaluated in a report titled “Pond Embankment Evaluation” prepared by GZA GeoEnvironmental of New York, dated February 20, 2015 for NRG-Huntley Power. This report was prepared prior to the EPA CCR Rule and included embankment assessments on multiple surface impoundments (ponds) located at the Site. The ponds evaluated included three ponds in the northern portion of the Site, two asphalt lined equalization ponds (for non-CCR process water) and the South Settling Pond. Since the CCR rule went into effect, the three northern ponds were closed and removed from service prior to the effective date of the CCR Rule and therefore are not addressed in this document. Additionally, as the two asphalt lined ponds adjacent to the South Settling Pond never contained or stored CCR associated wastes, these ponds are also not required to be addressed in accordance with the Design Criteria requirements of the CCR Rule.

October 14, 2016 Huntley CCR Surface Impoundment Structural Integrity

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In accordance with §257.73(f)(2), the use of a previously completed assessment(s) can be used as the initial assessments required by §257.73 as the previously completed embankment evaluation report is dated February 20, 2015 which meets the criteria of less than 42 months prior to October 17, 2016. Therefore, the following is a summary of the findings for the South Settling Pond that is supported by the February 2015 Pond Embankment Evaluation report. A copy of this report is attached to this document. §257.73(a)(2): Determination of an initial hazard potential classification for the CCR Surface Impoundment. Hazard Potential: LOW Per the requirements of §257.73(a)(2) an initial hazard potential classification assessment is required for the Site specific CCR surface impoundment and is to be classified as either high potential, significant potential or low potential hazard. Based on the conclusions presented in the previously completed Pond Embankment Evaluation report, the South Settling Pond was recommended to have a classification of “low hazard potential” since an improbable failure or mis-operation of the impoundment would result in no probable loss of human life and low economic and/or environmental losses. Additionally, since the time of the report, the discharge of CCR sluice water has been eliminated due to the Huntley Plant boilers being removed from service on February 29, 2016, resulting in a reduction of discharge flow rates from about 6,800 gallons per minute (gpm) down to about 1,500 gpm (for non-CCR water only). This flow reduction has resulted in a lowering of pond elevation from about elevation (el.) 570.0 to about el. 569.4 feet (as measured from the measurement gauge located near the outfall 008 pipe invert). This reduction in water elevation within the pond further supports the low hazard potential determination. §257.73(d): Completion of an initial structural stability analysis for the CCR Surface Impoundment Per the requirements of §257.73(d), the existing CCR surface impoundment must have an initial structural stability assessment completed to document, at a minimum, whether the CCR unit has been designed, constructed, operated and maintenance with the following requirements.

(i) Stability of Foundations and Abutments- Based on the previously completed Pond Embankment Evaluation report, the South Settling Pond embankment is identified as situated between the pond and the Niagara River and includes an asphalt pavement access road over its top portion. Rip rap armor (consisting of about 18-inch thick cut stone) was observed on the side slopes between the asphalt and the shorelines on both sides of the embankment. The rip rap located on the settling pond side has some grassy vegetation cover and the rip rap on the Niagara River side was observed and documented as interlocked with cement


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grout. This armor was reportedly installed over an approximate 12-inch layer of structural gravel and a woven fabric material over gradual fill soil and native silt and sand. A corrugated metal discharge pipe (CMP) is located through the embankment to allow drainage from the settling pond into the Niagara River. This CMP is oval shaped, with approximate dimensions of 65-inches tall by 92 inches wide and identified as Outfall #008. A plan view and cross section of the embankment and its associated materials are presented in Figure 3 and 4 of the attached Pond Embankment Evaluation. Weep drains consisting of approximate 4-inch diameter clay tile pipes were observed near the toe of the slope at 5-foot spacing and were installed to facilitate drainage of the River facing embankment side slope. The stability monitoring of the South Settling Pond embankment is generally limited to routine visual inspections for the side slopes, discharge pipe and grouted riprap side slopes. To date, there has been no evidence or indication of significant shifting, settling or general instability associated with the embankment.

(ii) Slope Protection- A review of the previous Pond Embankment Evaluation report indicates the structure was constructed with an approximate 18-inch think grouted rip rap armor (18-inch thick boulder sized rock) over 12-inches of structural gravel and a woven fabric. This slope protection is located on both the pond and river side of the embankment with the large CMP discharge pipe located in the central portion to drain the pond water into the river. The findings of the report, coupled with routine Site observations has not identified evidence of erosion or failures associated with the side slopes are indicative that the embankment rip-rap slopes area performing as designed. Based on this review and our observations, the embankment slope protection is providing sufficient protection from surface erosion, wave action and ice flows from the river, and from potential adverse effects of a rapid draw down

(iii) Dike Compaction – Minimal historical documentation/information/drawings were available pertaining to construction methodology (e.g., fill lift thickness, compactive effort, proctor values, type of equipment, etc.). However, test borings completed as part of the pond embankment assessment were used to assess subsurface soil conditions and identified the various soil units with the following soil properties/parameters (based on laboratory testing and test boring density values).

Soil Type Total Unit Weight

(pcf) Saturated Unit

Weight (pcf) Friction Angle

Riprap/Stone Layer (GW)

140 140 40

Fill Material (SM, SW, ML, GL)

128 130 30

Silt and Fine Sand Soil (ML, SM)

120.5 124.5 25

Sand (SW) 126 130 19


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(iv) Vegetated Slopes – The south settling pond embankment side slopes, both on the pond side and the river side, were constructed with a large rip-rap armor generally at a slope of 3 foot horizontal to 1 foot vertical (3H:1V). The river side slope was observed with grouted joints no vegetation and the pond side was observed with some grassy vegetation between armor block joints. Based on this design, the slope is considered to be an alternative form of slope protection and therefore is excluded from the vegetated slope requirements.

(v) Spill Way Configuration – The South Settling Pond was not designed with a spillway, rather it is equipped with a CMP discharge pipe located through the embankment that is sloped to allow drainage from the settling pond into the Niagara River. This CMP is oval shaped, with approximate dimensions of 65-inches tall by 92-inches wide which has been sized to allow drainage during design flood levels (assumed ½ probable maximum flood (PMF) or el. 571.4 ft for the South Settling Pond) while maintaining a sufficient freeboard of about 4 feet within the pond area. Assuming the outfall pipe remains unobstructed, the pond would not be able to overtop its banks. Since the embankment has no spillway and its sidewalls are constructed of a rip-rap armor, the embankment is excluded from the spillway requirements.

(vi) Hydraulic Structures – No hydraulic structures are present that underlie the base of the South

Settling Pond, however the CMP outfall #008 that discharges water from the pond to the Niagara River is routinely inspected by visual means for evidence of changes to its structural integrity, and has been historically identified as free of significant deterioration, deformation, distortion, bedding deficiencies, sedimentation and debris which may affect the operation of the hydraulic structure. As a result of the plant boilers being shut down on February 29, 2016, the residual CCR sluice water was effectively eliminated from being discharged into the pond by late March 2016. As a result, the remaining non-CCR water flow at the outfall pipe invert has been reduced from el. 570± to el. 569.25± resulting with a reduced wetted perimeter/flow within the pipe. Based on the findings of the routine visual inspections, coupled with the reduced flow within the outfall pipe, the structural integrity of the pipe, has been maintained and the structure is considered to be free of significant deterioration, deformation, distortion, bedding deficiencies, sedimentation and debris which may negatively affect the operation of the outfall structure.

(vii) Impacts from Downstream Waterbodies – The Niagara River is located downstream of the Huntley South Settling Pond at el. 566± during normal conditions as compared to the current pond elevations of slightly more than el. 569±. As stated in the Embankment Evaluation Report, a PMF has not been established for the Niagara River by a State, Federal or Canadian agency so the report used the 500-year elevation value of el. 570.65 as an example extreme flood condition for the Niagara River. This elevation is still below the berm height of el. 575.4 and would not overtop the berm to inundate the pond. Additionally, the design of the embankment slopes (grouted rip-rap armor and gravel) that comprise the slope face is


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designed, constructed and maintained to maintain structural stability of the embankment from impacts associated with the Niagara River.

Based on the reviewed information provided by the Huntley Plant, and on our understanding and observations made at the Site, significant structural deficiencies were not identified at the South Settling Pond Embankment of the Huntley Plant. Therefore, the design, construction, operation and maintenance of the of the Huntley South Settling Pond are consistent with recognized and generally accepted good engineering practice for the volume of CCR that remains at the Site (previously estimated at 23,000 cy±). We note that the surface impoundment no longer receives CCR wastes as the plants boilers have been removed from service and no longer generates CCR wastes. §257.73(e): Periodic Safety Factor Assessments for the CCR Surface Impoundment Per the requirements of §257.73(e), the existing CCR surface impoundment must have an initial safety factor assessment completed for the CCR unit and document whether the calculated factors of safety achieve the minimum safety factors specified in paragraphs (e)(i) through (iv) for the critical cross section of the embankment. As part of the previously completed Pond Embankment Evaluation report, a stability analysis was completed for the embankment in an effort to determine factors of safety for comparison to minimum EPA values for specific loading conditions (including normal pool, seismic conditions, rapid draw down and maximum pool (determined to be ½ PMF)) using the slope stability analysis program PCSTABL, Version 6 to provide an assessment of existing conditions at the Site. Additionally, soil parameters used in in the program were based on assumed soil index parameters that were based on soil test results and published values for similar soil types. A water level during normal conditions was assumed to range in elevation from about el. 566± (river elevation) to el. 570± (pond elevation) feet. The Pond elevation for ½ PMF was determined to be about el. 571.4± feet and the Niagara River elevation for the 500-year storm was determined to be el. 570.65 ft. The following factors of safety were calculated for specified loading conditions and compared to the EPA minimum required factors of safety.

Loading Condition Calculated F.S. EPA Minimum Required F.S.

Circular Normal Pool (No Seismic) 1.5 1.5

Circular Normal Pool (W/ Seismic) 1.1 1.0

Block Normal Pool (No Seismic) 1.6 1.5

Block Normal Pool (W/ Seismic) 1.3 1.0

½ PMF 1.6 1.4

Rapid Draw Down 1.3 1.3


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The safety factor calculated for the specific loading requirements were determined to meet or exceed the specific EPA minimum requirements. Based on these calculations and associated observations, GZA considers the South Settling Pond embankment along the Niagara River to be stable. PROFESSIONAL ENGINEER CERTIFICATION

The undersigned registered professional engineer is familiar with the CCR Surface Impoundment requirements of §257.73, specifically:

§257.73(a)(2) - Initial Hazard Potential Classification,

§257.73 (d) – Initial Structural Stability Assessment, and

§257.73 (e) – Initial Safety Factor Assessment and has reviewed available documentation specific to the Huntley Power CCR Surface Impoundment (referred to as the South Settling Pond). Based on the document review and Site observations made, the undersigned registered professional engineer attests that the Pond Embankment Evaluation report dated February 20, 2015 that was prepared for various impoundment structures at the Huntley Plant was conducted in accordance with and meets the specific requirements of §257.73(a)(2), (d) and (e). Name of Professional Engineer: Daniel J. Troy, P.E. Company: GZA GEOENVIRONMENTAL OF NEW YORK Signature: __________________________________ Date: October 14, 2016 PE Registration State: New York PE Registration Number: 081139-1 Professional Engineer Seal: We trust this information satisfies your needs for this project. Sincerely, GZA GEOENVIRONMENTAL OF NEW YORK Daniel J. Troy, P.E. Bart A. Klettke, P.E. Senior Project Manager Principal Attachments: Pond Embankment Evaluation, prepared for the Huntley Generation Plant, Tonawanda,

New York, prepared by GZA GeoEnvironmental of New York, dated February 20, 2015

A TTA C H M E N T A

535 Washington Street11th FloorBuffalo, New York14203716-685-2300Fax: 716-685-3629www.gza.com

GZAGeoEnvironmentalof New York

Engineers andScientists


February 20, 2015File No. 21.0056705.00

Mr. Joe [email protected] Power LLC3500 River RoadTonawanda, New York 14150

Re: Pond Embankment EvaluationHuntley Generation PlantTonawanda, New York

Dear Mr. Pietro:

GZA GeoEnvironmental of New York (GZA) is pleased to submit this Pond EmbankmentEvaluation Report to NRG / Huntley Power LLC (NRG) for three specific pondembankments located within the Huntley Generation Plant property at 3500 River Road inTonawanda, New York (Site). The three embankments assessed as part of this workinclude:

The embankment located between the South Settling Pond and the Niagara River in thesouthern portion of the Site (See Figure 1); and

Two embankments along the northern portion of the Site property associated withponds identified as Pond 2 and Pond 3 (see Figure 2).

This report summarizes:

The subsurface conditions encountered at each of the three specified pondembankments at the site based on the recently completed test boring program.

Topographic survey completed for the two northern embankments and the one southernembankment.

Analytical testing results for various soil samples collected during the soil boring work. Our geotechnical evaluation of each of the three embankments. An engineering evaluation and stability analysis for each of the three embankments. GZAs responses to AMECs comments pertaining to GZAs hydrologic and hydraulic

assessment of the Site as presented in our September 13, 2012 report.

BACKGROUND

The EPA has conducted nation-wide assessments of Coal Combustion Waste (CCW)impoundments at coal combustion energy producers. AMEC was hired by EPA to performassessments of six (6) ponds at NRG’s Huntley Site. AMEC’s June 2011 assessmentincluded a site visit to perform visual observations, inventory the CCW surfaceimpoundments, assess the containment dikes, and to collect relevant historical

Huntley Power, LLC. Page 2Pond Embankment Evaluation February 20, 2015

impoundment documentation. Condition assessments, as accepted by the National DamSafety Review Board (NDSRB), were ascribed by AMEC to each of the 6 impoundments,ranging from: “Satisfactory” – “Fair” – “Poor” – “Unsatisfactory” – “Not Rated”. AMECcompleted EPA’s Coal Combustion Dam Assessment Checklists and CCW ImpoundmentAssessment Forms. The Impoundment Inspection Forms include a section that assigned a“Hazard Potential” rating ranging from “Less than Low” – “Low” – “Significant” –“High”. Based on the AMEC reporting, the Ponds were given a draft rating of “Poor”mainly due to insufficient data.

NRG requested that GZA review the EPA/AMEC report to assist NRG in preparing aresponse letter to provide additional support for reclassifying the respective pondembankments. Based on this request, GZA prepared a letter response to AMEC datedSeptember 13, 2012 that provide additional rationale, analysis and general engineeringjudgments given the current site conditions. From this additional work, we provided ouropinion as to what the appropriate classification should be for the various impoundments,raising from Poor to Satisfactory, based on accepted EPA qualifiers or rankings.

EPA/AMEC responded to GZA’s response letter in a letter report dated March 2013 thatcommented on several aspects of our report. Overall, the Ponds 2 and 3 and the SouthSettlement pond were reclassified as Fair from the draft classification of Poor. AMECindicated that the classification of Fair and not Satisfactory was mainly due to the lack ofgeotechnical data available for the pond embankments. The letter stated that in an effortfor the respective ponds to be considered for a Satisfactory rating, geotechnicalinvestigations would be required to confirm strength parameters used in the stabilityanalysis. Additionally, as part of the geotechnical study, it was recommended that onepiezometer be installed at each embankment to monitor phreatic levels in the respectiveembankments.

AMECs comments also addressed several concerns regarding hydrologic and hydraulicrecommendations that required clarification of analyses prepared by GZA and also forannual observations of specific pond embankments to be made. We note that annualobservations have been completed by GZA for the years 2013 and 2014 with our findingsand recommendations report submitted to NRG under separate cover.

Subsequent to EPA/AMECs comment letter, NRG made the decision to drain and close thethree northern ponds at the Site prior to the end of 2014. Because these ponds only receivestorm water and some limited process water from the power plant facility, it wasdetermined that the generated water could be rerouted to another on-Site outfall fordischarge into the Niagara River and thereby eliminating the need for these northern ponds.These ponds are scheduled to be drained and filled with soil material (as approved by theNew York State Department of Environmental Conservation) prior to the end of 2014.Based on this information, much of the findings and clarifications recommended for thenorthern Pond 2 and Pond 3 may be considered moot due to their scheduled closure.


GEOTECHNICAL ASSESSMENT

In an effort to obtain additional strength parameters from the soil units within the threeponds, GZA was engaged by NRG to drill three (3) test borings at each of the threeidentified pond embankments to observe subsurface conditions and provide a geotechnicaland stability assessment of the respective embankments. Specific information obtained ateach pond embankment included fill type and thickness, overburden soil type, physicalproperties and thickness, and depth to groundwater. Existing discharge pipes are presentwithin each embankment, including one for Pond 3, one for the South Settling Pond andfive (of differing size and elevations) from Pond 2, which allows surface water to drainfrom the respective ponds to the Niagara River.

GZA completed the following scope of services for this work.

Retained the services of Earth Dimensions Inc. (EDI) of Elma, New York to completenine (9) test borings at the Site (see Figure 3 for South Settling Pond, Figure 5 forNorth Pond 2, and Figure 7 for North Pond 3 plan views). Three borings were done ateach pond embankment of which two were located on either side of the respectiveoutfall pipes including one closer to the pond, one closer to the outfall slope and one inthe middle of the embankment. The soil boring located closer to the respective outfallslope (discharge side) at each embankment was converted into a 2-inch diameterpiezometer to allow for groundwater measurements to be made within each respectiveembankment. Overburden soil samples were collected by EDI and GZA characterizedand logged the recovered soil samples. Ground water measurements were made fromwithin the completed piezometers, and when allowable, from drilling augers at thecompletion of the each boring.

Select overburden soil samples were collected for testing by GZA’s geotechnicallaboratory for various analyses including grain size (i.e., sieve tests), moisture content,total unit weight, direct shear and triaxial compression testing. To facilitate some ofthe testing, Shelby tube sample collection was attempted from distinct layers ofapparent fine grained soils and were submitted to our soils laboratory for consolidatedundrained triaxial testing and unit weight determination.

Ground surface elevations in the areas of the respective embankment areas weremeasured by GZA’s subcontractor, Clear Creek Land Surveying, LLC (Clear Creek) ofSpringville, New York. The ground surface elevation and location of Pond 2 and Pond3 embankments, at the critical embankment cross-sections, and the respective testborings and piezometers were recorded, as well as, existing embankment featuresincluding rip-rap location, respective pond water levels, concrete headwall (Pond 2only) and discharge pipe inverts, among others. These locations were tied into anexisting Site benchmark that was provided by NRG for our use. Measurements fromthe South Settling Pond were made by GZA using typical rod and level methods tocalculate elevations and tape measurements for horizontal measurements. Thesemeasurements were tied into the existing topographic survey completed by Clear Creekduring our 2009 embankment assessment.

Evaluated the stability of the three embankments for varying conditions (includingnormal pool, seismic conditions, rapid draw down and maximum pool (determined to


be ½ PMF) via the slope stability analysis program PCSTABL, Version 6 to provide anassessment of existing conditions at the Site.

Prepared this evaluation report that summarizes the findings of the subsurfaceexplorations, laboratory testing program, and embankment evaluation. Asrecommended by the AMEC response, this report also presents our recommendationsof whether or not a more detailed slope stability analysis of the embankment isrequired and/or what types of remedial efforts can be made to respective embankmentsto prevent slope failures under specified conditions.

SITE CONDITIONS

South Settling Pond Embankment

The South Settling Pond currently is designed to receive stormwater runoff and processwater associated with NRG’s bottom ash removal system. Specifically, this pond receivesflow from sluice waters and suspended solids from Unit 67 and Unit 68 bottom ash andeconomizer ash systems (only for a short duration in 2009) and discharge from the LowLevel Waste Water Pit. Since 2009, economizer ash has been removed from the facility dryand taken directly to the Huntley Landfill for disposal, and is no longer being sluiced to theSouth Settling Pond. The Low Level Pit discharge includes rain water from roadwaydrains, sub-basement sump drains, boiler water releases, Huntley 1 roof and floor drains,auxiliary cooling systems drains and discharge from the Wastewater Treatment facilityfrom treating the North and South Equalization basin water.

This ash is pumped to the settling pond, where the larger solids (e.g., bottom ash) beingdischarged settle out closer to the pipe discharge into the pond and the smaller particles(e.g., fly ash) settle out at distances further from the discharge pipes. Although thedischarge volume into the settling pond reportedly varies from time to time, the surfaceelevation of the water within the pond typically remains consistent at an approximateelevation (el.) 570, which is slightly above the invert of the discharge pipe inlet that drainsto the Niagara River. The settling pond is reportedly about 6 feet deep and is periodically(about once every five years) dredged to remove accumulated sediments (e.g., ash). Itshould be noted that accumulated ash proximate to the discharge pipes in the northernportion of the pond are dredged on a weekly basis to the extent practical. This weeklydredging of accumulated coarse grained ash reduces the accumulation of sediment withinthe central and southern portions of the settling pond.

This assessment area consists of the embankment located between the south end of thesettling pond and the Niagara River and includes an asphalt pavement access road over itstop portion. Rip rap armor was observed on the side slopes between the asphalt and theshorelines on both sides of the embankment. The rip rap located on the settling pond sidehas a grassy vegetation cover and the rip rap on the Niagara River side is interlocked witha cement grout with a small amount of vegetation present.

A corrugated metal discharge pipe (CMP) is present through the embankment that allowsdrainage from the settling pond to the Niagara River. This CMP is oval shaped, with


approximate dimensions of 65-inches tall by 95 inches wide. At the time of our visit, waterwas flowing through the pipe at an approximate depth of about 2 to 3 inches.

As shown on the attached Figure 4, the ground surface elevations of the existingembankment range from elevation (el.) 566 at the Niagara River shoreline (outside toe-of-slope) to el. 575.4 across the paved access road on top of the embankment to el. 570.0 atthe pond shoreline (inside toe-of-slope).

North Pond 2 Embankment

The general area around the existing Pond 2 area was originally designed to receive CoalCombustion Waste (CCW), and remaining areas surrounding the pond may containresidual ash from former plant activities. The pond currently receives flow from theadjacent Pond 1 which itself receives facility drainage including sub-basement sumppumps, roof and floor drains, auxiliary cooling system drains and de-mineralized waterproduction wastes.

Based on available pond design drawings (NRG No. 311.10-4578-01-0000), Pond 2appears to have been constructed within an area of former fly ash with the bottom of thepond consisting of a 12-inch thick layer of bank run gravel beneath a 6-inch thick layer ofreinforced concrete. The drawings imply the elevation of the bottom of Pond 2 is el. 570.0(top of reinforced concrete) and the eastern pond embankment as being constructed with anapproximate 6 foot wide clay soil wall that appears to extend below the finished pondfloor. The clay material was likely placed for its low permeability characteristics aroundthe pond sidewalls. The surface elevation of the water within Pond 2 can be controlled bya series of corrugated metal discharge pipes of varying elevations and sizes located withina concrete headwall structure on the northern side of the pond. Of the five outlet pipesobserved, only one pipe remains open at all times (overflow pipe) and the other four areequipped with slide gates to allow for moderation of pond levels. The open overflow pipehas the highest invert elevation at el. 577.1 ft. The elevation of Pond 2 water was designedfor and is typically maintained at el. 575. These five outlet pipes, which range in size from12-inch to 24-inch diameter, were observed to discharge into a stone lined open drainagechannel located about 45 feet north of the concrete headwall structure with invertelevations at the discharge points ranging from 568.9 to 573.0 ft. Discharge from thesepipes eventually flows westerly within the open drainage channel, into the Niagara River.

This assessment area consists of the embankment located between the north end of Pond 2and the stone lined open drainage channel. This embankment was generally observed tohave a soil/gravel access road over its top portion to allow for passage of service vehicles.A bank stabilization material consisting of a formed concrete-like blanket material(estimated 6-inches thick) was observed on both the pond side and the drainage channelside of the embankment. Additionally, loose rip-rap (boulders) was also observed placedalong portions of the embankment’s northern slope.

As shown on the attached Figure 6, the ground surface elevations of the existing Pond 2embankment range from el. 568 at the open channel drainage ditch (outside toe-of-slope)


to el. 579 across the access road on top of the embankment to el. 575 at the pond shoreline(inside toe-of-slope).

North Pond 3 Embankment

The general area of the existing Pond 3 area was originally designed to receive CCW, andremaining areas surrounding the pond may contain residual ash from former plantactivities similar to Pond 2. The pond is designed to receive flows from the adjacent Pond1. It should be noted that during recent visits, the drainage water was no longer flowinginto Pond 3 as the facility closed off the outfall pipe from Pond 1 to Pond 3 with a metalslide gate. As such, accumulated water in Pond 3 was too shallow to drain out through thenorthern outfall. This closure is likely associated with NRGs plan to drain and close thenorthern ponds.

Based on available pond design drawings (NRG No. 311.10-4578-01-0000), Pond 3appears to have been constructed within an area of former fly ash and does not appear tohave been constructed similar to Pond 2 (which included a reinforced concrete and stonebottom). The drawings imply the elevation of the bottom of Pond 3 is el. 566.0 ft. Thesurface elevation of the water within Pond 3 does not include control gates at its outfalllike Pond 2 (other than the ability to close off flow completely from Pond 1 slide gate) asthe northern end of the pond consists of an open 24-inch diameter CMP overflow pipe.The open drainage pipe has invert elevation at el. 574.2 on the pond side and el. 573.5 atthe outfall (open channel drainage ditch side). The elevation of surface water in Pond 3apparently varies, but appears to be designed for el. 575.0. Discharge from Pond 3eventually flows westerly within the open drainage channel into the Niagara River, withinthe same drainage channel as Pond 2.

This assessment area consists of the embankment located between the north end of Pond 3and the stone lined open drainage channel (about 80 feet in length). This embankment wasgenerally observed to have a soil/gravel access road over its top portion to allow forpassage of service vehicles. A bank stabilization material between the pond and thesouthern side of the Pond 3 embankment consisted of a formed concrete-like material(estimated 6 inches thick) was observed on the pond side of the embankment.

As shown on the attached Figure 8, the ground surface elevations of the existingembankment range from el. 571 at the open channel drainage ditch (outside toe-of-slope)to el. 578 across the access road on top of the embankment to el. 575 at the pond shoreline(design inside toe-of-slope).


SUBSURFACE EXPLORATIONS

The subsurface exploration program consisted of three test borings at each of the threeponds. The borings completed for the south settling pond were designated B-4 to B-6 as acontinuation from borings B-1 through B-3 that were previously done as part of our 2009embankment assessment. Borings for Pond 2 were designated as B-10 through B-12 andthose for Pond 3 were designated as B-7 through B-9. The test boring locations are shownon Figures 3, 5 and 7. General test boring procedures include the following.

Overburden drilling was done using 4-1/4 inch inside diameter hollow stem augers(HSA). In an effort to obtain accurate “N” values at locations below the groundwatertable, the drillers attempted to use a mud slurry in an effort to minimize potentialhydrostatic in-balance between water levels inside and outside the augers. It should benoted that only borings B-4 through B-7 were drilled with this slurry and the drillershad difficulty maintaining sufficient head within the augers. As a result, the remainingtest borings (B-8 through B-12) were drilled without the use of mud slurry.

Standard Penetration Tests (SPT) were completed in each boring in general accordancewith ASTM D1586. SPT “N” values were determined by driving a 2-inch diametersplit spoon sampler with a 140-pound automated hammer falling 30-inches. Soils weresampled over a 24-inch interval. The number of blows required to drive the split spoonsampler each 6-inch interval was recorded. The “N” value is the number of blowsrequired to drive the sampler between the 6-inch to 18-inch interval.

Split-spoon samples were recovered continuously to the bottom of each boring, at adepth of about 26 feet. The only exception was at boring B-12, which was done to adepth of 28 ft bgs.

Shelby tube samples were attempted from several test borings from potentiallycohesive soils, where encountered. With the exception of two locations, tube samplesrecovered typically had poor recovery/usability due to coarse grained soil (sand andsilt) sample sliding out on retrieval, sample interference from gravel or drillers wax orinsufficient recovery volume.

Recovered soil samples (including Shelby tube samples) that were collected during thiswork were shipped to our geotechnical laboratory for index and strength testing.

One test boring at each of the three embankments was equipped with a piezometer tomeasure water levels within the respective embankments. The piezometers wereconstructed of 2-inch diameter Schedule 40 PVC riser pipe and included a 10-foot slottedscreen. Steel protective casings were fitted and secured with concrete over the riser pipes.

Water level measurements were made inside the completed piezometers at differenttimes during the field work. Additionally, at completion of the borings, HSAs were leftopen overnight to allow water level measurements to be made inside the augers thenext morning.

GZA prepared test boring logs based on visual observations of the recovered soil samples,using apparent grain size distribution and plasticity. Characteristics such as relativedensity and consistency (based on the SPT), color, grain size, moisture, etc. were recordedon the boring logs. Test boring logs are included as Attachment 1.


TOPOGRAPHIC SURVEY

The location and ground surface elevation at each test boring completed from Pond 2 andPond 3 was measured in the field using GPS methods referencing the site datumestablished by NRG by GZAs subcontracted land surveyor, Clear Creek Land Surveying,LLC of Springville, New York. In an effort to properly tie into NRG’s site grid system,NRG was able to provide survey control points tied into NRG’s horizontal and vertical sitedatums. Additionally, Clear Creek was able to locate previous points used during their2009 survey measurements of the South Settlement Pond embankment. Locations andground surface elevation measurements for B-4 through B-6 at the South Settlement Pondembankment were made by GZA field staff using typical rod and level equipment andfiberglass tape based on known locations as identified on the 2009 topographic survey.Topographic surveys made for each pond embankment are incorporated into the respectiveFigures 5 and 7 for Ponds 2 and 3 respectively.

LABORATORY TESTING

After review of the boring logs, and collected soil samples and in consultation with NRG,GZA selected representative soil samples for laboratory testing to confirm fielddescriptions and to assist in estimating engineering properties of the silt and fine sand layerencountered within and beneath the embankment. The laboratory testing programconsisted of:

Seven (7) soil samples for grain size analyses (not including hydrometer testing)(ASTM D422);

Seven (7) soil samples for natural moisture content;

Additionally, a total of six (6) Shelby tube samples were collected from locations of finegrained soils and submitted for testing. Analyses of these samples included water content,total unit weight, torvane testing (shear strength), and where allowable, consolidatedundrained triaxial compression (CIU) tests (ASTM D4767).

It should be noted that only two Shelby tube samples (identified as ST-6 and ST-7 from B-8 and B-11, respectively) were identified as suitable for the CIU testing. The remainingtube samples generally had poor or insufficient recovery for this testing. A compositesample from tube samples ST-3 and ST-4 (from borings B-5 and B-6, respectfully) wasprepared by the laboratory to determine shear angle by a direct shear test (ASTM D3080).The tube sample ST-2 (from boring B-4) was observed with driller’s wax down the entirelength of the sample and therefore the calculated average total unit weight value reported isnot considered to be representative of the soil conditions and is not used in our stabilityanalysis.

The laboratory soil test results are included as Attachment 2.


SUBSURFACE CONDITIONS

The soil stratigraphy conditions observed are described in this section. A generalizedprofile between the test borings is depicted on embankment cross-sections Figures 4, 6 and8. The general thickness and elevations of the various soil layers encountered at the boringlocations are summarized below.

South Settlement Pond (Cross-Section on Figure 4)

In general, the soil borings B-4 through B-6 identified subsurface conditions generallysimilar to borings B-1 through B-3 that were done in the vicinity during our 2009investigation. Test boring B-4 was located proximate to the southern side of the dischargepipe in the western central portion of the embankment (closer to the Niagara River). Thisboring was equipped with a 2-inch diameter PVC piezometer. Boring B-5 was located southof the drainage pipe and adjacent to the pond and boring B-6 was located north of thedrainage pipe in the central portion of the access road. The ground surface elevation at B-4is approximately 575.2 feet above sea level (MSL), B-5 is approximately 575.4 feet MSLand B-6 is approximately 574.5 ft MSL. In general, the overburden conditions encounteredat the three locations explored are summarized as follows.

Overburden Fill: The fill thickness varied between test borings including 12.0 feet at B-4and B-5 and 10.0 feet at B-6. The soils sampled were visually described as varyingbetween sand, gravel and slag in the upper portions of the fill soil to silt and fine sandmaterial in the lower portions. Smaller amounts of brick, metal and wood fragmentswere observed throughout the fill material. The fill soil samples were predominantlycoarse grained and non-plastic and generally observed with relative densities rangingfrom very loose to very dense.

Silt and Fine Sand: The depth of the silt and fine sand soil encountered varied fromabout 12 feet bgs in B-4 and B-5 and to a depth of 10 ft bgs in B-6 and ranged fromabout 5.5 to 7.0 feet thick. The recovered samples were visually described as generally agrey to dark grey silt and fine sand soil (ML) with relative densities ranging from veryloose to loose.

Sand – A sand layer including very fine sand to coarse sand was observed at depthsranging from about 17.5 to 19 feet bgs and its presence continued to the end of eachboring (26 feet bgs). These sands were observed with trace amounts of silt and roundedgravel with relative densities ranging between very loose and dense.

Pond 2 (Cross-Section on Figure 6)

The three soil borings completed adjacent to Pond 2 designated as B-10 through B-12 areshown on Figure 5. Test boring B-10 was located proximate to the Pond, east of theembankment discharge pipes and concrete headwall (closer to pond). Test boring B-11 waslocated on the northern central portion of the embankment (east of the discharge pipes) and


was equipped with a of 2-inch diameter PVC piezometer. Boring B-12 was located west ofthe drainage pipes and headwall in the central portion of the embankment access road. Theground surface elevation at B-10 is approximately 579.6 feet MSL, B-11 is approximately579.0 feet MSL and B-12 is approximately 578.7 ft MSL. In general, the overburdenconditions encountered at the three locations explored within the embankment of Pond 2 aresummarized as follows.

Overburden Fill: The fill thickness varied between test borings including 16.0 ft at B-10,16.5 ft at B-11 and an apparent 28.0 feet at B-12. The soils sampled were visuallydescribed as varying layers and lenses of silty clay and fine sand and gravel (presumablycoal ash) in the fill soil. The silty clay soils fill was typically observed at shallowlocations is likely material used for construction of Pond 2 that was placed overpreviously deposited coal ash materials. Apparent coal ash waste and slag lenses wereobserved at the bottom of B-12 and may be due to historic activities prior to theconstruction of Pond 2.

Silt and Fine Sand: The depth of the silt and fine sand soil encountered varied fromabout 16.5 feet bgs in B-10, 16.0 ft bgs in B-11 and ranged from about 3 to 10.5 feetthick. The recovered samples were visually described generally as gray to black silt andfine sand soil with lesser amounts of clay and gravel.

Sand – A sand layer including fine sand to medium sand was observed at depthsranging from about 19.5 to 25 feet bgs and its presence continued to the end of eachboring. These sands were observed with trace amounts of silt and rounded gravel, withrelative densities ranging from very loose to loose.

Pond 3 (Cross-Section on Figure 8)

The three soil borings completed at this location, designated as B-7 through B-9, are shownon Figure 7. It should be noted that due to clearance issues, overhead electric lines prohibitedthe drillers from installing B-7 any further to the North. Based on NRG-provided site plans,these locations appear to be within the limits of the former Erie Canal right-of-way. Testboring B-7 was located proximate to the northern side of the Pond 3 embankment, east of thepond discharge pipe (closer to the drainage ditch) and was equipped with a 2-inch diameterPVC piezometer. Test boring B-8 was located along the eastern side of the discharge pipewithin the central portion of the embankment access road and B-9 was located on thewestern side of the Pond 3 discharge pipe at a location closer to the pond. The groundsurface elevation at B-7 is approximately 577.6 ft MSL, B-8 is approximately 578.1 ft MSLand B-9 is approximately 578.0 ft MSL. In general, the overburden conditions encounteredat the three locations explored within the embankment of Pond 3 are summarized as follows.

Overburden Fill: The fill thickness varied between test borings including 6.0 feet at B-7,9.5 feet at B-8, and 11.0 feet at B-9. The soils sampled were visually described as sandand gravel materials likely consisting predominantly of a coal ash waste material.Smaller amounts of silty clay, slag and rubber fragments were observed sporadically in


the fill material. The fill soil samples were predominantly coarse-grained and non-plasticwith relative densities ranging from very loose to medium dense.

Silty Clay (Fill): This silty clay soil is assumed to have been used to fill the historicErie Canal as a thin seam of gray to black f/c sand and gravel soil with traces of slagwas typically observed at depths of 17 to 18.5 feet bgs. The thin lens of coarse grainedmaterial is believed to be the bottom of the former Erie Canal. The depth of the reddishbrown silty clay (CL) ranged from 6.0 feet bgs in B-7, 9.5 feet bgs in B-8 and 11.0 feetbgs in B-9 and ranged from 8 to 12 feet thick. This historic fill soil had consistencyranging from soft to medium stiff.

Silty Clay (Native) – A reddish brown silty clay (CL) layer was observed at depthsranging from about 17 to 18.5 feet bgs (directly beneath the assumed bottom of historicErie Canal) and its presence continued to the end of each boring (26 feet bgs). Thismaterial is assumed to be a glacially deposited native soil (unlike the lacustrine silt andsand deposits generally observed closer to the Niagara River) as it was observed asmassive in structure and did not appear to have been placed as fill material. These soilshad consistencies ranging from soft to very stiff.

GROUNDWATER CONDITIONS

Groundwater was measured inside the three installed piezometers at various times and theobserved measurements are presented below.

Embankment TestBoring

Date ofMeasurement

WaterMeasurement

(ft bgs)

GroundwaterElevation (ftabove MSL)

SouthSettlement Pond

B-4 6/30/14 and 8/5/14 11.3 / 11.3 566.5 / 566.5

Pond 3 B-7 6/30/14 and 8/5/14 6.7 / 7.3 573.9 / 573.3Pond 2 B-11 6/30/14 and 8/5/14 13.2 / 14.6 568.7 / 567.3

The water elevation observed in the Niagara River adjacent to the South Settlement Pondoutfall pipe appeared similar to the elevation observed during our 2009 assessment (basedon location of shoreline to clay tiles, outfall pipe, rip rap stone, etc.). Based on theseobservations, GZA has assumed an elevation of the Niagara River to remain at 566.0 ftMSL (which corresponds to the water elevation measured in the piezometer of B-4).

As part of our September 13, 2012 response letter to AMEC, hydrologic analysis forspecific ponds located at the Site were made. At AMECs recommendations, design floodlevels for the three ponds was assumed to be ½ Probable Maximum Flood (½ PMF). Thecalculations and design programs used by GZA to calculate the respective ½ PMF for thethree ponds is presented in our previously submitted response letter. Flood elevationsutilized in this report’s slope stability assessment include the following.


Location Normal Pool El. (ft. MSL) ½ PMF El. (ft. MSL)Pond 2 575.3 577.2Pond 3 574.4 577.4

South Settling Pond 569.9 571.4Niagara River 566.0 570.7*

*Represents tailwater water elevation during 500-year peak flood level for the NiagaraRiver as no PMF elevation was available for the Niagara River.

SLOPE STABILITY ANALYSES

South Settlement Pond Conditions

The soils encountered in B-4 through B-6 generally consists of a fine to coarse grained fillmaterial over a silt and fine sand layer over a well graded sandy soil. At the boringlocations, the composition of the fill material was variable with a greater amount of coarsesoil (sand, gravel and slag and lesser amounts of concrete, brick, metal and wood debris)noted closer to the ground surface. Finer grained, sandy silt soil was observed generally inthe lower portions of the fill layer. The soil encountered below the fill and below the waterline was predominately a loose silt and fine sand (apparent lacustrine deposited soil) about5.5 to 7 feet thick over a well graded medium dense sandy soil. The findings from theseborings are generally similar to those of B-1 through B-3 from our 2009 investigation.

SPT “N” values from the silt and fine sand layer underlying the fill soils (about 10 to 12feet bgs) were measured with values ranging from about 3 to 8 indicating a loose relativedensity for the silt layer and from 9 to 31 indicating a loose to dense relative density for thefine sand layer).The average relative densities per boring ranged from 12 to 33 indicating amedium dense fill material (generally similar to the previous borings B-1 through B-3).

The rip rap side slopes extending upward from the edge of the river and pond to the top ofthe embankment are generally observed to be sloped at 3-feet horizontal (H) to 1-footvertical (V).

Pond 2 Conditions

The soil encountered in B-10 through B-12 generally consists of a fine to coarse grainedfill material over a sandy silt layer over a fine sand layer. At the boring locations, thecomposition of the fill material was generally observed as a variable consistency with agreater amount of fine grained soil (i.e., silty clay) noted closer to the ground surface(presumed placed during pond construction) and fine sand and gravel (presumably coalash/waste) in the lower portions of the fill soil. The soil encountered below the fill andbelow the water line was predominately a very loose to loose silt and fine sand (apparentlacustrine deposited soil) about 2 to7 feet thick over well-graded medium dense fine sand.


SPT “N” values from the silt and fine sand layer underlying the fill soils were measuredwith values ranging from about 2 to 8 indicating a loose relative density in locations B-10and B-11. The soil Boring B-12 (western most boring in the Pond 2 embankment) hadvery loose conditions with low SPT “N” values (including weight of rod measurements) atdeeper depths and may be historic fill material placed in this area prior to construction ofthe pond.

The embankment side slopes were observed consisting of a concrete bank stabilizermaterial on both sides. Large rip rap stone was also observed on the northern slope facingthe drainage channel. An approximate 12 foot deep reinforced concrete headwall with sidewingwalls was also observed as part of this embankment. The foundation footers for thisstructure were reportedly about 18 feet long and 16 feet wide. This headwall included fiveseparate discharge pipe openings at varying elevations designed to control pond waterelevations. It should be noted that stability modeling did not take into account this structureor the associated CMP drainage pipes with varying elevations. These existing features arelikely to increase the calculated stabilities of the Pond 2 embankment.

Pond 3 Conditions

The soils encountered in soil borings B-7 through B-9 completed within the embankmentfor Pond 3, identified a similar coarse-grained soil as the other two embankments.However, these soil borings appear to be located within the former Erie Canal. The fillsoil within this embankment was observed to be similar coarse grained material intermixedwith coal ash waste. The soil encountered below the granular fill and below the water linewas predominately an apparent fill material likely used to fill the former canal consistingof soft to very stiff silty clay (about 8 to 12 feet thick) over an assumed native, soft to stiff,silty clay.

SPT “N” values from the silty clay layers (both above and below the presumed bottom ofthe former Erie Canal) underlying the shallow granular fill soils were measured withvalues ranging from about 2 to 26 indicating a soft to stiff consistency in the three borings.The relative densities of the granular fill soil ranged from 3 to 28 indicating loose tomedium dense consistency.

The embankment side slope proximate to the Pond was observed with a concrete slopestabilization material similar to Pond 2. This material was not readily observed on thenorthern slope adjacent to the drainage channel, mostly due to dense vegetation (grass andweeds) obstructing its observation. Large loose rip rap stone was observed, however, onportions of the northern slope.


GZA EVALUATION OF TRIAXIAL COMPRESSION TEST RESULTS

GZA attempted to collect Shelby Tube samples of the fine grained soil layers encounteredat each embankment in an effort to determine an internal friction angles () and total unitweight for each location. Specifically, tube samples were attempted within the silty soillacustrine deposit layers of the South Settlement Pond and from Pond 2. The silty clay filland native glacial deposit layers were attempted from Pond 3. However, due to poorsampler recovery or insufficient sample volume at select locations, only two Shelby tubesamples (from B-8 and B-11) were able to be tested for tri-axial compression testing. Ourinterpretation of the completed test indicates the following.

South Settlement Pond B-5 and B-6: A composite sample from B-5 and B-6 (ST-3 and ST-4, respectfully) was prepared by the laboratory due to insufficient sample volume and/orsuitability of recovered sample. Based on this composite sample, an internal friction angleof 19 degrees was calculated.

Pond 2, B-6 (ST-6): A stress path aligned tangential to the stress circles plotted for failureat low minor principal inter-granular stress (3) and high 3 produces a angle of 29degrees with a shear strength intercept (cohesion (c)) of about 500 psf.

Pond 3, B-11 (ST-7): A stress path beginning at the plot origin (shear strength = 0 psf) andextending tangentially to the stress circle for failure at high 3 produces a angle of 38degrees.

GZA analyzed the respective embankment’s stability considering these calculated frictionangles results.

SLOPE STABILITY EVALUATION

Using the computer program PCSTABL (version 6) to analyze the stability of the slopeembankment at the study area and our assumed soil index parameters (which are based onsoil test results, published values for similar soils and based on our experience withpreviously tested Site soils) presented in the tables below provides an analysis thatindicates that the three embankment slopes are stable.

For previous seismic analyses, GZA applied a maximum horizontal acceleration (MHA) of0.2g (90 percent probability of not being exceeded in 250 years), based on “ProbabilisticEarthquake Acceleration and Velocity Maps for the United States and Puerto Rico”, U.S.Geologic Survey, Map MF-2120. This is a conservative value based on publishedinformation. More recent published data, which has catalogued earthquake activity,indicates lower MHA values. Additionally, based on the findings of the document“Geotechnical Engineering Circular No. 3 – Design Guidance: Geotechnical EarthquakeEngineering for Highways, Volume 1 – Design Principals” dated May 1997 and publishedby the U.S. Department of Transportation, seismic acceleration values for slopes andembankments can be estimated to be one half the value of the maximum horizontal


acceleration. Based on this information, therefore, GZA applied a horizontal accelerationvalue of 0.1 g (half of 0.2g) for the slope stability calculations, for each pond embankmentseismic evaluation.

Our stability analyses considered circular and block failures. The circular failures werecalculated with and without seismic conditions, with assumed ½ PMF conditions and withrapid draw down conditions for the respective embankment. The sliding block failure wascalculated with and without seismic conditions. Slopes are generally considered to bestable with factors of safety greater than 1.5 for normal conditions (without seismiceffects), and greater than 1.0 for normal conditions with seismic considerations. Aminimum factor of safety for conditions of maximum surcharge pool (½ PMF for thisstudy) are 1.4 and the minimum value for rapid draw down from a maximum storage poolis 1.3.

Our analysis for each specific pond embankment is further discussed below.

SOUTH SETTLEMENT POND

SOIL PARAMETERS USED FOR PCSTABL INPUT

Soil TypeTotal Unit

Weight(pcf)

SaturatedUnit Weight

(pcf)

FrictionAngle

Riprap/Stone Layer (GW) 140 140 40

Fill Material (SM, SW, ML, GL) 128 130 30

Silt and Fine Sand Soil (ML,SM)

120.5* 124.5 25

Sand (SW) ** 126 130 19

*A laboratory test for total unit weight for the silt and fine sand layer was attempted fromsample identified as ST-2 from B-4, however the sample result was deemed unusable asdrillers wax extended down the entire length of the Shelby tube sample. Based on this, theunit weights previously determined from the 2009 laboratory sampling of similar soil layerwere used.** Due to poor sample recovery, the friction angle for the sand layer was calculated bycompositing soil samples from Shelby Tubes ST-3 and ST-4 (from borings B-5 and B-6,respectfully). The average total unit weight was based on an average from both samples.


GZA ascribed the same unit weight (128 pcf) and friction angle (30o) for the Fill materialas previously estimated for the following reasons.

1. The relative densities were observed to be similar to those of previous borings B-1through B-3, generally observed as medium dense.

2. The 30o friction angle value assigned is roughly based upon internal friction anglevalues published for silty-sand fill by Joseph E. Bowles, “Physical andGeotechnical Properties of Soils”, 1979, as follows: Loose Silty Sand: 25-35degrees; Dense Silty Sand: 30 – 36 degrees. Based on these published values, anascribed 30o friction angle is appropriate.

3. Due to the presence of gravel, slag, concrete, brick, cobbles and wood debris in thefill soils, plus the presence of the 65” x 92” steel arch pipe providingreinforcement, it is GZA’s opinion that the debris and pipe gives greaterinterlocking and a higher shear strength that warranted assigning a mid-rangefriction angle of 30 degrees to the fill layer.

A piezometric (water) level during normal conditions was assumed to range in elevationfrom about 566 (river elevation) to 570 (pond elevation) feet. The Pond elevation for ½PMF was previously calculated to be about 577 feet and the Niagara River elevation forthe 500 year storm was determined to be 570.65 ft. The following factors of safety werecalculated for specified loading conditions. Copies of the different run models are includedin Attachment 3.

Loading Condition Calculated F.S EPA Minimum RequiredF.S.

Circular Normal Pool (No Seismic) 1.5 1.5Circular Normal Pool (W/ Seismic) 1.1 1.0Block Normal Pool (No Seismic) 1.6 1.5Block Normal Pool (W/ Seismic) 1.3 1.0½ PMF 1.6 1.4Rapid Draw Down 1.3 1.3

Based on these calculations and associated observations, GZA considers the South AshSettling Basin embankment along the Niagara River to be stable.


NORTH PONDS

POND 2


Soil TypeTotal Unit

Weight(pcf)


(pcf)

FrictionAngle

Riprap/Stone Layer (GW) 140 140 40

Fill Material (SM, SW, ML, SP) 122 126 28

Silt, Clayey Silt and Fine Sand Soil(ML, SM)*

113 117 38

Sand (SW)** 128 132 19

*Values for this layer based on lab results collected from Shelby Tube sample ST-7 (fromB-11).**Values for this layer are based on sample results obtained from the sand layer associatedwith South Settling Pond due to similar conditions observed at similar depths.

A piezometric (water) level during normal conditions was assumed to range in elevationfrom about el. 566 (river elevation) to el. 575 (Pond 2 elevation) feet. The Pond Elevationfor ½ PMF was previously calculated to be about el. 577.2 feet and the Niagara Riverelevation for the 500 year storm was determined to be el. 570.65 ft, which is slightly higherthan the ground surface of the adjacent drainage channel (el. 568 ft MSL). The followingfactors of safety were calculated for specified loading conditions. Copies of the differentrun models are included in Attachment 3.

Loading Condition Calculated F.S EPA MinimumRequired F.S


With the exception of the ½ PMF loading condition, the factors of safety calculated usingupdated laboratory and strength parameters exceed the EPA minimum required safetyfactors. GZA acknowledges that the value of 1.1 for ½ PMF is less than the required valueof 1.4, however the failure plane for this condition appears to be limited to a shallowveneer failure of the rip rap and bank stabilization material. Potential failure along this


plane would not be catastrophic to the embankment and associated Pond 2. Additionally,the calculated FS values are likely lower than actual as our slope stability model does nottake into account for the added stability and strength of the reinforced concrete headwall,associated wing walls and broad-width footer, and numerous CMP drainage pipes whichvary in elevation throughout the embankment. Furthermore, due to NRGs plan to drainand fill Pond 2 prior to the end of 2014, the issue of embankment stability for Pond 2becomes moot.

Based on these calculations and associated observations, GZA considers the Pond 2embankment to be stable. However, as this pond is scheduled to be drained and filled, theissue of stability will not be applicable.

POND 3


Soil TypeTotal Unit

Weight(pcf)


(pcf)

FrictionAngle

Fill Material (SM, SW, ML, GL) 122 126 28

Silty Clay Fill (CL)* 125.4 128 29

Silty Clay Native (CL)** 133.5 135 29

*Total Unit Weight values for this layer are based on laboratory results collected fromShelby Tube sample ST-5 (from B-7). Friction angle was not tested although appears to besimilar soil to the apparent native soils below it. Hence, a friction angle value of 29 wasused.** Values for this layer based on lab results (including friction angle) collected fromShelby Tube sample ST-6 (from B-8).

A piezometric (water) level during normal conditions was assumed to range in elevationfrom about 569 (bottom of adjacent drainage channel) to 574 (Pond 3 elevation) feet. ThePond Elevation for ½ PMF was previously calculated to be about 577.4 feet and theNiagara River elevation for the 500-year storm was determined to be 570.65 ft, which isslightly higher than the ground surface of the adjacent drainage channel (569 ft MSL). Thefollowing factors of safety were calculated for specified loading conditions. Copies of thedifferent run models are included in Attachment 3.


Loading Condition Calculated F.S EPA MinimumRequired F.S


With the exception of the ½ PMF loading condition, the factors of safety calculated usingupdated laboratory and strength parameters exceed the EPA minimum required safetyfactors. GZA acknowledges that the value of 1.3 for ½ PMF is less than the required valueof 1.4, however the failure plane for this condition appears to be limited to a shallowveneer failure of the rip rap and bank stabilization equipment. Potential failure along thisplane would not be catastrophic to the embankment and associated Pond 2 as the top ofthis embankment is about 70 feet wide. Furthermore, due to NRGs plan to drain and fillPond 3 prior to the end of 2014, the issue of embankment stability for this pond becomesmoot.

Based on these calculations and associated observations, GZA considers the Pond 3embankment to be stable. However, as this pond is scheduled to be drained and filled, theissue of stability will not be applicable.

HYDROLOGIC AND HYDRAULIC STUDY CLARIFICATION

GZAs responses to AMECs comments (shown in italics) pertaining to GZAs hydrologicand hydraulic assessment of the Site as presented in our September 13, 2012 report areaddressed below.

AMEC comment: “The 72-hour All-Season Probable Maximum Precipitation (PMP)generated by BOSS HMR52 specifically for this site is 33.0 inches and reasonable. Thematerial submitted does not indicate the drawdown time of the impoundments to determineif a longer design storm duration is necessary due to long storage time in the ponds.”

GZA Response: As Pond 2 and Pond 3 are scheduled to be closed and filled in, there is noneed to evaluate whether a longer design storm duration is necessary. Additionally, due tothe size of the South Settling Pond outfall pipe (96-inches) and its ability to maintain morethan 3-feet freeboard during ½ PMF conditions, it is our opinion that there will be no needto evaluate for longer storm design durations.

AMEC comment: “The report states that the ½ PMF was generated by taking 50% of thecalculated discharge from application of the PMP to each watershed. The full model oroutput was not provided, so it is not known how this reduction was done in HEC-HMS.

The 500-year flood elevation on the Niagara River was used as the tailwater condition forthe pond H&H analysis. It is incorrectly labeled on the drawings accompanying the report


as the ½ PMF water surface elevation for the river. The 500-year flood event is notdirectly related to the PMF, and the ½ PMF flow in the Niagara River could be larger orsmaller than the 500-year flow. There may be a determination of the PMF for the NiagaraRiver performed by a State, Federal, or Canadian agency. If it is possible to obtain a PMFflowrate for the river it could be compared to the flow rates given in the effective FIS todetermine if the ½ PMF flow is similar to the 500-year flow. If no estimate of the PMFexists for the Niagara River, using the 500-year elevation as an example extreme floodcondition is defensible, especially as it falls approximately 2.7 feet lower than the lowestoutlet of the North Basins, 1.1 feet lower than the bottom of the equalization basins and 8.6feet lower than the overflow outlet, and South Ash Settling Pond has over 3 feet offreeboard even with the tailwater condition.”

GZAs Response: It is GZA’s current understanding that a PMF has not been establishedfor the Niagara River by a State, Federal or Canadian agency. After an additional reviewfor such information, we have opted to use the 500-year elevation value of 570.65 ft MSLas an example extreme flood condition for the Niagara River.

AMEC comment: “The HEC-HMS models were not provided for review, and no outputwas included with the report. This review is based only on the given input and outputtables in the report and the attached routing diagram. No drainage area maps wereprovided. The elevations of the various pipe inlets and outlets and tops of the berms aregiven in the report and the attached drawings. These have been assumed to be correct andentered into HEC-HMS model accurately, but this can’t be verified.”

GZA Response:

The HEC-HMS models are included on a compact disc that has been provided to NRG,separate from this report.

Drainage area maps for the south ponds and the north ponds are attached as Figures 9 and10, respectively. Elevations of the pipe inlets and outlets and top of berm elevations at theoutlet structures have been measured by our surveyor (Clear Creek) as shown on attacheddrawings.

AMEC comment: “The Equalization Basins drainage area is equal to its total pondfootprint of 132,400 (sq. ft.), which matches up with the aerial photo, and indicates that norainfall runoff from adjacent areas will surface drain into it. The drainage area of theSouth Ash Settling Pond is 343,700 (sq. ft.), which is larger than the pond footprint andindicates some adjacent area flowing to the pond. Page 6 of the report states that someroadway and building areas flow to this pond. The drainage area for this pond should bedelineated on a map for review.

Case B, considering the tailwater impacts of Ponds 2 and 3 on Pond 1 is the mostappropriate scenario to consider for the North Basins. It shows freeboard of at least 2.1feet in each pond. The Conclusion section of the report (Pages 17-19), however, showsdifferent ponding elevations for each pond that does not match the values in the results


table (Table 8, Page 12). The freeboard values from the conclusion are less than 2 feet forPonds 2 and 3. No model or detailed output was submitted to verify the results presented inthe report and to determine which freeboard values are correct”.

GZA Response: GZA has delineated the pond drainage areas as shown on attached Figures9 and 10.

The text presented in the Conclusions section of our September 2012 report wasincorrectly stated. The text, specific to the North Basins is corrected as follows to matchthe values presented in the tables and figures.

Correction to Page 17 of September 2012 GZA Report, under “North Basins” discussion inConclusions (corrected items in italics): “Pond 1 – This pond is small, covering an arealess than ½-acre, with partial embankments (Top El. 579.0’ +) between itself and Ponds 2and 3. The hydrologic analysis indicates that the ½ PMF event would result in a peakstorm water elevation of 576.9’ providing about 2.1 feet of freeboard height.”

Correction to Page 17 of September 2012 GZA Report, under “North Basins” discussion inConclusions (corrected items in italics): “Pond 2 – This pond has a full surroundingembankment (Top El. 579.0’ +). The hydrologic analysis indicates that the ½ PMF eventwould result in a peak storm water elevation of 576.40’ providing about 2.6 feet offreeboard height.”

Correction to Page 18 of September 2012 GZA Report, under “North Basins” discussion inConclusions (corrected items in italics): “Pond 3 - This pond has partial embankments(Top El. 579.0’ +) along the west and north edges, with the east and south sides incised.The hydrologic analysis indicates that the ½ PMF would result in a peak storm waterelevation of 576.5’ providing about 2.5 feet of freeboard height.”

The HEC-HMS models are presented on a compact disc that has been provided to NRGseparate from this report.

AMEC comment pertaining to Freeboard Guidelines for the south-side EqualizationBasins: “The Equalization Basins have zero or less freeboard throughout many operatingscenarios, even with the suggested plan to lower the overflow outlet pipe.”

GZA Response:

Section 4.2 of the March 2013 AMEC letter acknowledges that the North and SouthEqualization Basins were determined to not be CCW impoundments (and were thereforenot rated by AMEC). As such, we believe that these equalization basins should not besubject to the freeboard guidelines as stated in Chapter 8, Section 9 of the MSHA CoalMine Impoundment Inspection and Plan Review Handbook. However, if these equalizationbasins are required to comply with the stated freeboard guidelines, an additional freeboard(minimum 3 feet high) may be attained atop designated lengths of the existing perimeterberms of the Equalization Basins, as shown on attached Figure 9. Adding freeboard atop


these berm intervals would prevent overflow to the Niagara River and direct the flow to theSouth Settling Pond. A cross-section view of this potential addition is shown on attachedFigure 9.

CONCLUSIONS

SOUTH PONDS

South Settling Pond

Our current geotechnical investigations and laboratory testing results generally confirm ourprevious estimates of soil strength parameters. It is our opinion, therefore, that the SouthSettling Pond should have a National Dam Safety Review Board (NDSRB) conditionassessment of “Satisfactory”1 in that no existing or potential embankment safetydeficiencies are recognized for all loading conditions.

In the highly unlikely event of a rare or extreme hydrologic and/or seismic event resultingin an embankment deficiency, the resultant risk of uncontrolled flow to the Niagara Rivercould be quickly mitigated by the following procedures.

1. Shutting off the process water influent to the South Settling Pond.2. Temporarily damming off the narrow section (about 60 feet wide) of the South

Settling Pond immediately upstream of the outlet pipe using clay soils readilyavailable in the area.

3. Establishing a temporary process water bypass system to decant the water to theNiagara River downstream of the temporary dam.

4. Repairing the embankment and restoring normal South Settling Pond operations.

We also believe that the South Settling Pond should have a “Low Hazard Potential” sincean improbable failure or mis-operation of the impoundment results in no probable loss ofhuman life and low economic and/or environmental losses. NRG would experience theeconomic loss of repairing the embankment deficiency; low environmental loss may beexperienced for the short duration in shutting off the process water feeding the SouthSettling Pond and establishing a temporary dam and bypass system described above. Lowenvironmental loss would also be attributed to the fact that NRG dredges the majority ofcoal combustion waste (CCW) sediment at the north-side inlet end of the South SettlingPond about 1,200 feet upstream of the pond outlet to the Niagara River. Transport ofsignificant amounts of CCW sediment over that distance is unlikely to take place whenNRG would immediately implement process inflow shut-off, temporary damming andbypass operations described above. The amount of CCW sediment formerly transportedfrom NRG’s Unit 67 and Unit 68 bottom ash and economizer ash systems discharged fromthe Low Level Waste Water Pit to the South Settling Pond has been greatly reduced sinceeconomizer ash is currently removed dry and taken directly to the Huntley Landfill, and is

1 NDSRB Condition Assessment “Satisfactory” definition: No existing or potential dam safety deficienciesare recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic)in accordance with the applicable regulatory criteria or tolerable risk guidelines.


no longer being sluiced to the South Settlement Pond. Economizer Ash was only sent tothe South Settlement pond for a short duration in 2009. This reduction in sedimenttransport into the South Settling Pond reduces the potential for sediment release to theNiagara River.

GZA recommends that periodic inspection and maintenance of the grouted rip rap and claypipe drains be made on an annual basis. Areas of damaged grout between the rip rapshould be filled and the clay pipes should periodically be cleared of accumulated riverdebris. Additionally, the existing corrugated metal drainage pipe located between thesettling pond and the Niagara River should periodically be inspected and maintained freeof accumulating debris to allow for proper pond drainage.

Another option for NRG to consider, if they determine it economically feasible, would beto implement a redundant outlet berm or containment system. An additional permanentcontainment structure (e.g., earthen berm, steel sheet pile coffer dam, etc.) with outlet flowstructure could be constructed immediately upstream of the outlet pipe, across the narrow60-feet wide section of the basin. The new outlet pipe could be equipped with a gatestructure for immediate shut-off capability, if needed in the unlikely event of failure of theexisting berm. Location of this new berm would not significantly impact the storagecapacity of the South Settling Pond.

Equalization Basins

AMEC stated in their March 2013 comment letter that the North and South EqualizationBasins were determined to not be CCW impoundments and are, therefore, not rated.AMEC did state, however, that our estimated freeboard height of less than 1 foot for the ½-PMF flood condition is less than the minimum 3 feet minimum freeboard guideline asstated in Chapter 8, Section 9 of the MSHA Coal Mine Impoundment Inspection and PlanReview Handbook.

It is our opinion that since the Equalization Basins are not considered to be CCWimpoundments, their minimum freeboard height should not be governed by the above-stated MSHA guidelines. However, if determined economically feasible by NRG, NRGmay consider constructing additional berm height to the designated sections of theperimeter berms as shown and detailed on attached Figure 9, to attain a minimumfreeboard height of 3 feet.

NORTH PONDS

Based on NRG’s planned closure of the North Ponds 1, 2 and 3 to be completed prior tothe end of 2014, the consideration for a satisfactory rating will not be required once theyare drained and backfilled. However, the current slope and subsurface conditions measuredand evaluated for the existing embankments generally indicates that the embankments arestable. Surficial erosion, due to the potential undercutting of the slope by various methods(e.g., Niagara River Ice or wave action, animal burrows, construction activities, etc.,) didnot appear to be an issue based on our field investigations and observations. The rip rap /


stone veneers on the various embankments appear to be suitably drained and secured. It isour opinion, therefore, that the Northern Pond 2 and Pond 3 have a Satisfactory rating forthe remaining time the Ponds are utilized by NRG for retaining water.

We appreciate the opportunity to have completed this work for NRG / Huntley PowerLLC, We will contact you in a few days to discuss this report and address any questions orcomments you may have.

Sincerely,

GZA GEOENVIRONMENTAL OF NEW YORK

Daniel Troy, P.E.Senior Project Manager

Bart Klettke, P.E.Principal

Attachments: Figures 1 through 10Attachment 1 – Subsurface Boring LogsAttachment 2 – Laboratory Test ResultsAttachment 3 – Slope Stability Model AnalysesHEC-HMS Models on CD (Provided to NRG under separatetransmittal)

FIGURES

92" x 65" STEEL PIPE ARCH

INVERT ELEVS.:

INLET: 568.94'

OUTLET @ NIAGARA RIVER: 568.04'

TOP OF BERM ELEV. OVER PIPE: 575.4'

NORTH EQUALIZATION

BASIN (POND #1)

BOTTOM ELEV. 571.8'

SOUTH EQUALIZATION

BASIN (POND #2)

BOTTOM ELEV. 572.3'

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PROCESS

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MAXIMUM FLOW:

6,800 GPM

EQUALIZATION BASINS

PIPE OVERFLOW OUTLET TO SOUTH ASH

SETTLING BASIN, INV. ELEV. @ 579.3'. MAXIMUM

PUMPED FLOW INTO BASINS = 500 GPM + STORM

CONTRIBUTION WITHIN CENTERLINES OF BERMS

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NORMAL POOL EL. 569.9'

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EL. 571.4'

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NORMAL POOL EL. &

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PMF LEVEL VARIES

(CONTROLLED BY PUMPS)

MAY 2009 SURVEY MEASUREMENT)

500-YR FLOOD TAILWATER EL. 570.7'

A

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SEE

FIGURE 3

GZA GeoEnvironmental Inc.Engineers and Scientists

NRG OCTOBER 2014 RESPONSE TO EPA REPORT

NRG HUNTLEY PLANT

TONAWANDA, NEW YORK

NRG HUNTLEY POWER, LLC

3500 RIVER ROAD

SOUTH PONDS

SITE PLAN

OCTOBER 2014 21.0056705.00

1

535 WASHINGTON STREET 11th FLOOR

BUFFALO, NEW YORK 14203

(716) 685-2300

0 X X

NOT TO SCALE

TONAWANDA, NEW YORK

POND 2

POND 1

POND 3

ESTIMATED

INVERT ELEVS.:

OUTLET @

NIAGARA RIVER:

MULTIPLE SIZED CMP

DISCHARGE PIPES AT

VARYING ELEVATIONS

(SEE FIGURE 5)

EXISTING DRAINAGE

CHANNEL, INV. EL. @

POND 2

43" x 27" GALV. PIPE ARCH'S

POND 1 TO POND 2:

INLET ELEV.: 576.1'

OUTLET ELEV.: 575.4'

POND 1 TO POND 3:

INLET ELEV: 576.1'

OUTLET ELEV.: 575.7'

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PONDS IS GRANULAR COAL ASH

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INTERIOR ELEVATED

FLOOR AREA OF POND 3

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PUMPED PROCESS FLOW INTO

POND 1 (ASH SLUICING WATER &

LOW VOLUME WASTES). MAX.

BOAT

MARINA

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FIGURE 8

GZA GeoEnvironmental Inc.Engineers and Scientists


NRG HUNTLEY PLANT

TONAWANDA, NEW YORK


3500 RIVER ROAD

NORTH PONDS 1-3

SITE PLAN

OCTOBER 2014 21.0056705.00

2



(716) 685-2300

0 X X

NOT TO SCALE

TONAWANDA, NEW YORK

A

A

B-4

B-6

B-5

N

0 10 20 40

SCALE IN FEET

GZA GeoEnvironmental of N.Y.Engineers and Scientists


NRG HUNTLEY PLANT

TONAWANDA, NEW YORK


3500 RIVER ROAD

TONAWANDA, NEW YORK

OCTOBER 2014 21.0056705.00

3



(716) 685-2300

SOUTH SETTLING POND

BORING LOCATION PLAN

SHEET NO.

GZA GeoEnvironmental of N.Y.Engineers and Scientists

PREPARED BY:PREPARED FOR:

www.gza.com

PROJECT NO.DATE: REVISION NO.

DESIGNED BY:

PROJ MGR:

DRAWN BY:

REVIEWED BY: CHECKED BY:

SCALE:


NRG HUNTLEY PLANT

TONAWANDA, NEW YORK

SOUTH SETTLING PONDCROSS SECTION


3500 RIVER ROAD

TONAWANDA, NEW YORK

OCTOBER 2014 21.0056705.00

FIGURE

4

4 OF 10

DJT

RJS

BAK

RJS

DJT

1" = 10'

0 5 10 20

SCALE IN FEET

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Overhead

Utilities

Conc Bottom

of Headwall

Elev: 569.99

Bollard

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Benchmark

In Headwall

579.79'

Valve

Inv 24" CM Pipe:

573.02

Inv 12" CM Pipe:

571.72

Inv 12" CM Pipe:

568.87

Inv 12" CM Pipe:

569.59

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october 14, 2016 [email protected] huntley...

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